The invention relates to a fuel metering system for a turbojet engine.
A fuel metering system is used in the case of the EJ 200 engine in the Jager 90/EFA airplane. Here, the pressure control device is constructed as an independently operating pressure control valve influenced only by the differential pressure at the main metering valve. The emergency device is formed by a separate emergency valve controlled by the engine controller. In the event of an unintentional opening of the main metering valve, the emergency valve is controlled by the engine controller so that, as a result of recirculation, the fuel amount reaching the engine is limited. However, the opening of the emergency valve causes a simultaneous closing of the automatic pressure control valve which aims at keeping the pressure drop at the main metering valve constant. Thus, the pressure control valve compensates the effect of the emergency valve during the change-over operation from the normal to the emergency operation. During this "dead time", as a result of an excess of fuel until the effective intervention of the emergency valve, there will be rotational speed overshoots of up to 15%. These have to be taken into account when designing the stability of the rotating engine components, causing these to be heavier and more voluminous.
In contrast, it is an object of the invention to provide a fuel metering system which avoids relevant rotational speed overshoots and thus permits an engine design which is more favorable with respect to its stability and weight.
This object is achieved according to the present invention.
The essence of the invention is the fact that the pressure control function for the main metering valve and the emergency function in the event of an unintentional opening of the main metering valve are combined in a single valve, here called recirculation valve, both functions being completely under the control of the engine controller, so that disadvantageous compensation effects are avoided. A secondary effect is the saving of components and weight.